California Avocado Society 1952 Yearbook 37: 154-158
AN APPROACH TO THE EVALUATION OF AVOCADO ROOTSTOCK
VARIABILITY
F. F. Halma and E. F. Frolich
Professor of Subtropical Horticulture, and Laboratory Technician,
Subtropical Horticulture, University of California.
SUMMARY
1. A method by which a limited number of cuttings can be obtained from very young
avocado seedlings is described and also discussed in relation to its application to
rootstock research.
2. A method, less limited in scope and already reported on by the junior author, is
briefly discussed.
An unavoidable shortcoming of avocado rootstock research is the use of seedlings as
rootstocks. Avocados do not reproduce true from seed, hence, in effect, every budded
or grafted tree is on a different variety. Whatever these differences in seedling
rootstocks may be, and to what extent they affect the scion variety, is difficult to
determine. Soil variability and other orchard conditions which cannot be controlled
complicate the problem. However, the use of cuttings would eliminate or minimize the
genetic variability in root-stocks.
Investigation by Eggers and Halma (1) in 1937 showed that cuttings from mature
Mexican trees could be rooted, but the percentage was generally low and variable.
Guatemalan failed to root and Fuerte, a so-called hybrid, gave very poor results.
However, cuttings from very young seedlings (if Mexican and Guatemalan rooted very
well. Of course such cuttings are of questionable value as rootstocks because nothing is
known of the parent seedling.
Nevertheless this finding was applied in the present investigation, the primary purpose
of which was to find out how many cuttings can be obtained from individual seedlings,
and secondly, to study growth variability and subsequent effect on the scion, provided a
sufficient number of progeny of individual seedlings can be obtained.
The following experiment was set up in 1950. Twenty-five seeds of each of 4
Guatemalan varieties—Anaheim, Dickinson, Itzamna, and Nabal— and 4 Mexican
varieties—Blake, Mexicola, Northrop, and Topa Topa— were planted in sand and
placed in a glasshouse to hasten germination and to insure rapid growth. Five to 7
seedlings of each variety, selected at random, were used as mother plants (Fig. 1).
The first cuttings were taken when the seedlings were about 6 inches high. Only one
cutting, about 3 inches long, was obtainable because there were then no laterals. The
removal of the growing point resulted in the production of laterals which furnished the
second lot of cuttings. This procedure was continued until it appeared that the mother
plant was no longer able to regenerate new suitable material (Fig. 2). It should be
mentioned here that some of the parent plants died as a result of this operation but that
most of those that survived eventually developed into sturdy plants.
l
The cuttings, all succulent with leaves intact, were rooted in vermiculite in a covered
propagating box located inside a glasshouse. High humidity was maintained to prevent
wilting. Practically 100 % rooted within a few weeks. Fig. 3 shows a typical root system.
Two to 5 lots of cuttings were obtained from each seedling, and, as table 1 shows, the
total number of rooted cuttings from seedlings of each variety varied. Also shown is the
relative weight of seed for each variety to indicate that seed size was not related to the
number of cuttings yielded during the five-month period. For example, Nabal and
Mexicola yielded the same number of cuttings, although the seed of the former weighed
about 47 grams and the latter only 18 grams.
The rooted cuttings were first planted in 3-inch pots and later transferred to larger paper
pots and placed in a lathhouse. Figure 5 shows representative plants about 18 months
after the cuttings were removed from the mother plants. Of interest is the fact that the
growth rate was about the same for Guatemalan and Mexican varieties, although
seedlings of the same Guatemalan varieties grow faster than Mexican, at least for the
first year.
Another observation of interest is that the response to rooting was most pronounced for
the first two lots. The third and fourth lots did not root as rapidly as the first and second.
Most of the cuttings were planted in 2 plots during May and June 1952. The remaining
ones were transferred to 3-gallon cans and kept in reserve. They were from 18 to 22
months old from the time of their removal from the propagating frame. One plot is
located at the Citrus Experiment Station, Riverside, in cooperation with G. A. Zentmyer,
and one in the Santa Barbara County Hospital grounds, in cooperation with (Farm
Advisor G. E. Goodall. It should be remarked here that the original plan of making a
single planting in order to have a greater number of progeny under similar conditions did
not materialize beans? a large enough area suitable for this purpose could not be found.
The Riverside plot consisted of 59 Guatemalan and 60 Mexican cuttings averaging 3 for
each of 40 parents. The Santa Barbara planting included 71 Guatemalan and 114
Mexican, averaging 4 for each of 36 parents. In addition 28 parent plants were planted.
The present plan is to allow the trees to develop undisturbed for at least two years, in
order to obtain information concerning variability. The Riverside plot is then to be used
for disease inoculation experiments and the Santa Barbara trees for top-working to a
commercial variety.
In concluding the report on this investigation, it should he emphasized that this method
of propagating rootstocks has no practical application. It merely furnishes genetically
uniform material which is essential in obtaining information concerning the range of
variability among seedlings used as rootstock. It may also be used in testing relative
disease susceptibility and in procuring several progeny of hybrid seedlings as insurance
against loss of the mother plant. If more glasshouse space were available, more parent
seedlings could be used. It would then be possible to obtain more parents yielding at
least ten or more rooted cuttings.
A method less limited in scope and offering greater possibilities in rootstock research
than the one just reported on, should be mentioned. It has recently been developed and
published by Frolich (2). Essentially it consists of obtaining rooted cuttings from stems,
the bases of which have at no time been exposed to light, that is, roots will develop
more readily in tissue in which chlorophyll formation is prevented. There are other
necessary conditions that have to be met which are described in the report, hence need
not be repeated here.
The principal advantage of this method lies in the fact that rooted cuttings can be
obtained in any numbers from mature trees of standard varieties. Frolich mentions only
three Guatemalan varieties, but since then cuttings of several additional varieties and
several species of Persea have been rooted. The percentages in all cases have been
very high, probably averaging 90 percent. However, it should be mentioned that the
rooted cuttings require greater care after their removal from the parent plant than
cuttings obtained from young seedlings. Figure 4 shows an 8-month-old cutting
obtained by this method. Several of them have been planted in rootstock plots for
comparison with budded trees on various stock varieties. With adequate glasshouse
space a sufficient number of rooted cuttings could be obtained for use as rootstocks in
test plots. This would eliminate or minimize variability due to the genetic factor in
rootstock trials.
LITERATURE CITED
1. Eggers, E. R., and F. F. Halma. Rooting avocado cuttings. Calif. Avocado Assn.
Yearbook 1937: 121-125.
2. Frolich, E. F. Rooting Guatemalan avocado cuttings. Calif. Avocado Society
Yearbook 1951: 136-138.